The use of plastics has expanded significantly in recent years, in part because they are inexpensive, easily molded into desired shapes, electrically non-conductive, lightweight and offer long service life. The joining of plastics is crucial to the more widespread use of these materials and, although plastics have been joined for many years, there is a need for new welding techniques that are simpler, faster and less expensive. Plastics can be joined by mechanical fasteners, adhesives or welding. Mechanical fasteners are common and are suitable for most materials but can be expensive and may not provide a permanent (when desirable) joint or adequate properties. Adhesives give good properties and sound joints but can be difficult to handle, require care with joint and surface preparation, and are slow. Welding is ideally suited to use in the mass production industries as the techniques are economical, simple, fast, reliable and capable of making joints with static mechanical properties close to those of the parent material.
One conventional technique for joining plastic parts together is the use of hooks and latches which interlock when pressed together. However, those very features needed to effectively attach one plastic part to another now make the mold tool more complicated and expensive. Intricately shaped plastic parts are more expensive than simply shaped ones because their molds include tools that must be sequentially removed from the finished part in multiple directions.
Thermoplastic parts may be temporarily joined by press fitting where a shaft is pressed into a boss of substantially the same shape but slightly smaller dimensions. However, after a period of time these parts are prone to separate--especially when subjected to moisture, temperature variations, vibration or frequent handling. To overcome this problem, manufacturers use solvents to soften thermoplastic parts which are then clamped or pressed together until the solvent evaporates. Similarly, the use of adhesives or bonding materials generally require long curing periods and contribute to air pollution. Long curing periods are inconsistent with modern manufacturing techniques such as just-in-time fabrication used to reduce factory inventory and lower costs. Although the evaporation rate of a solvent or the curing rate of a bonding material may be hastened by heating, it is desirable to simplify, further shorten, and thereby cost reduce the process.
A survey of techniques for permanently joining plastics is disclosed in a paper entitled "Welding Plastics for the Automotive Industry" by Martin N. Watson, SAE Technical Paper Series 860581, Feb. 24-28, 1986. The disclosed techniques include: (i) Ultrasonic Welding--a bonding process that utilizes high frequency mechanical vibrations. The parts to be assembled are held together under pressure and then subjected to ultrasonic vibration at right angles to the contact area so that longitudinal vibrations are transmitted through the component. Although ultrasonic welding is well suited to mass production and offers high speed, tooling is expensive. (ii) Spin Welding--friction heating due to spin, angular or orbital rotation of thermoplastic material to cause melting/welding to occur. A disadvantage of the spin welding process is that in its simplest form, it is only suitable for applications in which at least one of the components is circular and requires no angular alignment. (iii) Vibration Welding--frictional heating generated by the relative movement between two parts to be welded which are held under pressure. Once molten material has been generated at the joint interface, vibration is stopped, the parts are aligned and the weld solidifies on cooling. Weld times of 1-5 seconds are possible with applied vibrations being typically 100-240 Hz, 150 mm amplitude, and the pressure of 1-4N/mm.sup.2. This technique is frequently used for the welding of various mass produced thermoplastic components. While these various techniques are generally useful, it is desirable to further decrease weld time, and to simplify the equipment needed to carry out the welding process.